Junction temperature prediction method and apparatus for use in a power conversion module

ABSTRACT

A method and apparatus for predicting junction device temperature of at least a first switching device in a power conversion module that includes a plurality of switching devices, the method comprising the steps of, during switching activity, identifying at least one operating characteristic of the first switching device and solving an equation that uses the identified operating characteristic to predict the temperature of the first switching device where the equation solved is a function of the location of the first switching device with respect to the other switching devices in the plurality.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 11/238,005which has the same title and was filed on Sep. 28, 2005 now U.S. Pat.No. 7,356,441.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

The present invention relates to integrated gate bipolar transistor(IGBT) modules and more specifically to a method and apparatus forpredicting the junction temperatures of IGBTs in an IGBT moduleoperating at a low frequency or a DC condition.

Because of their advantageous operating characteristics (e.g., highswitching speeds) IGBTs are used in many different types of powerconditioning modules including AC to DC converters, DC to AC inverters,AC-DC-AC converters, etc. For example, in the case of a DC-AC inverter,six IGBTs are arranged to form an inverter bridge along with six diodes.

During switching operations IGBTs and diodes generate heat that has amagnitude related to the switching frequency as well as the amount ofcurrent passing through the devices. When IGBT or diode junctiontemperature exceeds a specific threshold temperature associated with adevice type, the devices have been known to fail. In order to reducefailure rate, IGBTs and diodes used in power conditioning modules aretypically mounted to heat dissipating devices such as air or liquidcooled heat sinks and are rated for specific current levels/switchingfrequencies.

It is not possible to measure the temperature of a diode or IGBTjunction directly and therefore device junction temperature has to beestimated or predicted. To predict device junction temperature duringswitching operations, some industry members have identified the thermalimpedance associated with each device type and have mounted atemperature sensor (e.g., a negative temperature coefficient sensor (NTCsensor)) to the device (e.g., to a device case as opposed to at thejunction itself). Then, during device switching, the measured devicetemperature and thermal impedance are used to calculate the power lossesof the device and hence to predict the IGBT junction temperature.Hereinafter, the method described above to predict junction temperaturewill be referred to as a conventional prediction method. This methodworks well in cases where switching devices (e.g., IGBTs, diodes) arethermally isolated from other switching devices (i.e., where devices aremounted on separate heat sinks or are separated by a substantialdistance (e.g., three device width dimensions) from other devices on thesame sink).

To reduce the space required by the switching devices and diodes as wellas the number of heat sinking components, in many cases a single heatsink having a single mounting surface is provided where all of the IGBTsand diodes that comprise a conditioning circuit are mounted to thesingle mounting surface. Unfortunately, when devices are mounted inclose formations on a single heat sink, the conventional predictionmethod described above has been shown to be inaccurate. In the case oftightly packed devices on a single sink, because one device is extremelyclose to other devices on the sink, heat form one device tends to heatup adjacent devices. While heat from one device tends to increase thetemperature of adjacent devices under all operating conditions, theneighbor heating effect is exacerbated at low switching frequencies andwhen a conditioning circuit is operated under DC conditions. Forinstance, in at least some experiments it has been observed that underDC conditions in a six-pack IGBT inverter module, a maximum predictionerror of nearly 30 degrees Celsius has occurred when using theconventional prediction method.

In order to avoid device failure due to the prediction error, onesolution has been to rate conversion modules (e.g., inverter,converters, etc.) at lower current and switching frequency levels (i.e.,are de-rated) than the separate switches used to configure the modules.While this solution substantially eliminates the failure problem, thissolution is relatively expensive as circuits including larger and morecostly switching devices are required for specific current levels andswitching frequencies. In addition, because the switching devices arephysically larger, the sinks for mounting the devices are larger and theoverall space required to accommodate the conversion modules isincreased.

BRIEF SUMMARY OF THE INVENTION

It has been recognized that the accuracy of a junction temperatureprediction algorithm can be increased substantially by accounting forthe effects of at least some inter-module switching device heating aswell as the effects of other ambient heating characteristics. Thus, thepresent invention includes methods and apparatus that relativelyaccurately predict switching device junction temperature by accountingfor at least a subset of heating characteristics of adjacent devices andat least a subset of module impedances.

At least some inventive embodiments include a method for predictingjunction device temperature of at least a first switching device in apower conversion module that includes the first switching device and atleast a second switching device, the method comprising the steps ofidentifying a cross thermal impedance value indicative of how thetemperature of the second switching device effects the first switchingdevice temperature and using the cross thermal impedance value topredict the temperature of the at least a first switching device.

In some cases the module includes, in addition to the first and seconddevices, a plurality of additional switching devices and wherein themethod further includes the steps of identifying a cross thermalimpedance for each of the plurality of additional devices indicative ofhow the temperature of the additional device effects the first switchingdevice temperature and using all of the cross thermal impedance valuesto predict the temperature of the at least a first switching device.

In some embodiments the cross thermal impedance between the second andfirst devices is a first impedance value and the module includes, inaddition to the first and second devices, at least a third switchingdevice and wherein the method further includes the steps of identifyinga second cross thermal impedance that is indicative of how thetemperature of the third device effects the first device temperaturewhere the first and second cross thermal impedance values are different.

In some cases the method further includes the step of identifying a selfthermal impedance value associated with the first switching device, thestep of using the cross thermal impedance including mathematicallycombining the self thermal impedance and the cross coupling impedance topredict the first switching device temperature.

In some embodiments the method further includes the steps of providing atemperature sensor at least proximate the module, identifying at leastone coupling thermal impedance from the second switching device to thesensor and generating a temperature value via the sensor, the step ofusing the cross thermal impedance including the step of mathematicallycombining the cross coupling impedance, the at least one couplingthermal impedance to the sensor, the temperature value generated by thesensor and the self impedance of the first switching device to predictjunction temperature of the first switching device. In some cases eachof the switching devices are one of a diode and an IGBT.

Some embodiments include a method for predicting junction devicetemperature of at least a first switching device in a power conversionmodule that includes a plurality of switching devices, the methodcomprising the steps of during switching activity, identifying at leastone operating characteristic of the first switching device and solvingan equation that uses the identified operating characteristic to predictthe temperature of the first switching device where the equation solvedis a function of the location of the first switching device with respectto the other switching devices in the plurality. In some cases theoperating characteristic is the power loss of the first switchingdevice.

In some cases the equation accounts for power losses of switchingdevices adjacent the first switching device as well as cross thermalimpedance between the adjacent devices and the first switching deviceand wherein the method further includes the steps of identifying a crossthermal impedance between a switching device adjacent the firstswitching device and the first switching device and, during switchingactivity, identifying the power losses of switching devices adjacent thefirst switching device.

In some cases the switching devices adjacent the first switching deviceare neighboring devices and wherein the equation accounts for powerlosses of switching devices adjacent the neighboring devices as well ascross thermal impedance between the devices adjacent the neighboringdevices and wherein the method further includes the steps of identifyinga cross thermal impedance between a switching device adjacent aneighboring switching device and the first switching device and, duringswitching activity, identifying the power losses of switching devicesadjacent the first switching device.

Some embodiments include a method for predicting junction devicetemperatures of at least a subset of devices in a power conversionmodule that includes a heat sink that forms a mounting surface, six IGBTdevices and six diode devices that are mounted to the mounting surface,the method comprising the steps of identifying cross thermal impedancevalues indicative of the effect that temperature of adjacent deviceshave on each other, during switching activity, identifying power lossesof at least a subset of the devices and using the device power lossvalues and the cross thermal impedance values to predict thetemperatures of at least a subset of the module devices.

In some cases the step of identifying power losses includes identifyingpower losses of each of the module devices and wherein the step of usingthe power loss values and the cross thermal impedance values includesusing the values to predict the temperatures of each of the moduledevices.

In some embodiments the method further includes the steps of providing atemperature sensor at least proximate the module and sensing thetemperature of the module via the sensor, the step of using the valuesto predict the temperatures of at least a subset of the module devicesincluding also using the sensed temperature value.

In some cases the method further includes the step of identifyingthermal coupling impedances between the sensor and at least a subset ofthe devices, the step of using the values to predict the temperatures ofat least a subset of the module devices also including using the thermalcoupling impedances.

In some cases the method further includes the step of identifying a selfimpedance value for each of the module devices and, during switchingactivity, identifying power losses for each of the devices, the step ofusing the values to predict the temperatures also including using theself impedance values and the power losses of each of the devices.

In addition, some embodiments include an apparatus for use with a modulethat includes a plurality of electronic switching devices, the apparatusfor predicting the junction temperature of at least a first of theswitching devices, the apparatus comprising a processor that runs aprogram to perform the steps of identifying power loss of at least asecond of the switching devices using the power loss value of the atleast a second switching device to predict the temperature of the firstswitching device junction.

In some cases the apparatus further includes a database in which isstored a cross thermal impedance value indicative of the thermalimpedance between the first and second devices, the processor using thepower loss value and the cross thermal impedance to predict thetemperature of the first switching device junction.

In some cases the database includes cross thermal impedance values for asub-set of the devices in addition to the second device that areindicative of the thermal impedance between each of the other devicesand the first device, the processor further identifying power lossvalues for each of the sub-set of devices and using all of the powerloss values and the cross thermal impedance values to predict thetemperature of the first switching device junction.

These and other objects, advantages and aspects of the invention willbecome apparent from the following description. In the description,reference is made to the accompanying drawings which form a part hereof,and in which there is shown a preferred embodiment of the invention.Such embodiment does not necessarily represent the full scope of theinvention and reference is made therefore, to the claims herein forinterpreting the scope of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic view of a three phase inverter;

FIG. 2 is a perspective view of inverter IGBTs and diodes mounted to amechanical heat sink;

FIG. 3 is a schematic diagram illustrating an equivalent circuit of thethermal impedance between a device junction and a case layer for whichmanufacturers typically provide values;

FIG. 4 is a schematic diagram similar to FIG. 3, albeit illustrating anequivalent circuit that includes additional components that account forself-impedance of devices;

FIG. 5 is similar to FIG. 3, albeit illustrating an equivalent circuitrepresenting coupling thermal impedance from a neighbor device;

FIG. 6 is similar to FIG. 3, albeit illustrating an equivalent circuitrepresenting coupling thermal impedance from an inverter device to a NTCtemperature sensor;

FIG. 7 is a graph illustrating experimental and curve fitted devicetemperatures for diode D3 in FIG. 2 when diode D3 is powered;

FIG. 8 is similar to FIG. 7, albeit illustrating curves corresponding toswitch S4 in FIG. 2 when diode D4 is powered; and

FIG. 9 is similar to FIG. 7, albeit illustrating curves corresponding toswitch S3 when switch S4 is powered.

DETAILED DESCRIPTION OF THE INVENTION

Development of the Temperature Predicting Model

Referring now to the drawings wherein like reference numerals correspondto similar elements throughout the several views and more specifically,referring to FIG. 1, the present invention will be described in thecontext of an exemplary three phase DC to AC inverter type powerconditioning module 10. Module 10 includes six IGBT switch devices S1-S6and six diodes (also generally referred to as switching devices) D1-D6,a separate diode linked to each of the IGBTs in inverse parallelrelationship. The IGBT switches are linked in series pairs acrosspositive and negative DC buses and a central node of each pair is linkedto a separate phase of a three phase load 12 (e.g., motor). As known inthe power conversion art, by switching the IGBTs, three phase voltagedelivered to load 12 can be controlled.

Referring still to FIG. 1, current sensors 23 are linked to the motorsupply lines for, as the label implies, sensing line currents. An NTCtemperature sensor 24 is mounted proximate at least one of the switchingdevices for measuring a temperature proximate the devices. A processor29 receives signals from the current sensor and NTC sensor 24 and usesthe received signals to predict temperatures of the inverter devices.

Referring to FIG. 2, a perspective view of a six-pack inverter module 14is illustrated which includes a mechanical heat sink 16, three mountingsubstrates 18, 20 and 22, IGBTs S1-S6, diodes D1-D6 and the NTCtemperature sensor 24. The sink includes a flat mounting surface 26.IGBT pairs and associated diodes are each mounted via a separate one ofthe substrates to the mounting surface 26. For example, IGBTs S1 and S2and associated diodes D1 and D2 are mounted to surface 26 with substrate18 between the devices S1, S2, D1 and D2 and the sink mounting surface26, devices S3, S4, D3 and D4 are mounted to surface 26 with substrate20 between the devices and surface 26, and so on. As illustrated,substrates 18, 20 and 22 are arranged in a single row so that substrate20 is between substrates 18 and 22. NTC sensor 24 is located inside acorner of module 14. Although not illustrated, one or more fan modulesmay be provided adjacent the rear sink surface that forms fins tofacilitate sink cooling activity.

In order to accurately predict the temperature of each module device,the effects of various heat sources on device temperature have to beaccounted for. The following discussion develops equations for takinginto account all of the heat sources that affect device temperatureduring switching activity.

The thermal impedance between a device junction and a device case foreach IGBT or diode can be physically represented by four parallel RCsub-circuits which, unless indicated otherwise, will be referred to as“layers” hereinafter. The four layers can be arranged in series asillustrated in FIG. 3 to transform the physical representation to amathematical representation. Note that after the transformation as shownin FIG. 3, the “layers” of the sub-circuit become meaningless. In FIG.3, the thermal resistance and time constant values corresponding to theR and C components are typically provided by device manufacturers.Exemplary thermal resistances and time constants for each of the fourlayers are shown in table 1 that correspond to a EUPEC FS150R12KE3 powerconversion module.

TABLE 1 Layers 1 2 3 4 R_(dk) 0.14283 0.17143 0.01931 0.00341 t_(dk)0.06499 0.02601 0.002364 1.187e−5 R_(ik) 0.07559 0.09061 0.01039 0.00341t_(ik) 0.06499 0.02601 0.002364 1.187e−5

From FIG. 3, the following mathematical equations can be formulated toexpress the thermal impedance associated with a single device in a powerconversion module:ZI _(jc) =R _(i1) //C _(i1) +R _(i2) //C _(i2) +R _(i3) //C _(i3) +R_(i4) //C _(i4)  Eq. 1ZD _(jc) =R _(d1) //C _(d1) +R _(d2) //C _(d2) +R _(d3) //C _(d3) +R_(d4) //C _(d4)  Eq. 2where ZI_(jC) and ZD_(jc) are the junction to case thermal impedance ofan IGBT device and a diode device respectively, R_(ik) and C_(ik) arethe thermal resistance and capacitance of the k_(th) layer of each IGBTdevice, respectively, and R_(dk) and C_(dk) are thermal resistance andcapacitance of the k_(th) layer of each diode device, respectively.

Since the IGBT and diode devices are directly mounted on an air cooledheat-sink in the present example, the temperature increase of the caseshould be considered when predicting the temperature of each device.Referring to FIG. 4, it has been recognized through experiment that thethermal impedance between the interface plane of the case and theambient can be physically approximated by two additional R-Csub-circuits or layers. A sink layer RC sub-circuit including R_(L5) andC_(L5) represents a temperature increase in thermal grease associatedwith the device to sink interface planes and the aluminum of the heatsink. An ambient layer RC sub-circuit including R_(L6) and C_(L6)represents the temperature increase between the interface plane of theheat sink and the plane of the ambient (i.e., between the sink and theair beneath the sink. Thus, a more complete self thermal impedance ofthe IGBT and diode devices mounted on a sink can be mathematicallyrepresented by the circuit shown in FIG. 4 and the self impedanceZII_(ij) between the i_(th) IGBT from the junction to the ambient andthe self thermal impedance ZDD_(ii) of the i_(th) diode from thejunction to the ambient can be expressed by the two following equations:ZII _(ii) =ZI _(jc) +ZII _(5ii) +ZII _(6ii)  Eq. 3ZDD _(ii) =ZD _(jc) +ZDD _(5ii) +ZDD _(6ii)  Eq. 4where ZI_(jc) and ZD_(jc) are the thermal impedances between thejunction and case layer for each IGBT and diode individually (seeEquations 1 and 2 above), ZII_(5ii) and ZDD_(5ii) are the thermalimpedances of the sink layer for the i_(th) IGBT and i_(th) diode,respectively, and ZII_(6ii) and ZDD_(6ii) are the impedances of theambient layer (i.e., the subscripts “5” and “6” correspond to the 5thand 6th RC layers in FIG. 4).

The IGBT and diode devices in a power conversion module are mounted tothe sink in very close proximity to each other and therefore thermalcoupling between devices must be considered to accurately predict devicetemperature. Through experimentation it has been recognized that thecoupling thermal impedance between IGBT and diode devices on a sink canbe physically approximated by two additional layers represented byparallel RC sub-circuits. One is the sink layer which represents thetemperature increase from case to sink across the thermal grease andfrom grease to ambient across the heat sink. The other is the ambientlayer that represents the temperature increase of the air beneath theheat sink. Thus, the coupling thermal impedance between an IGBT or diodedevice and a neighboring device can be mathematically represented asillustrated in FIG. 5 including series RC sub-circuits including R_(L1)and C_(L1) and R_(L2) and C_(L2). The coupling ZII_(ij) impedance fromthe i_(th) IGBT to the j_(th) IGBT and the impedance ZID_(ij) from thei_(th) IGBT to the j_(th) diode device can be expressed by the followingequations:ZII _(ij) =ZII _(5ij) +ZII _(6ij)  Eq. 5ZID _(ij) =ZID _(5ij) +ZID _(6ij)  Eq. 6where, ZII_(5ij) and ZID_(5ij) are the coupling thermal impedances fromthe i_(th) IGBT to the j_(th) IGBT and the j_(th) diodes at the sinklayer, respectively, and ZII_(6ij) and ZID_(6ij) are the couplingthermal impedances from the i_(th) IGBT to the j_(th) IGBT and thej_(th) diodes at the ambient layer, respectively. Generally, thecoupling thermal impedance between two chips decreases when theirdistance increases.

Similarly, the coupling thermal impedance from the i_(th) diode to theother module devices can also be expressed by the following equationsZDI _(ij) =ZDI _(5ij) +ZDI _(6ij)  Eq. 7ZDD _(ij) =ZID _(5ij) +ZID _(6ij)  Eq. 8where, ZDI_(ij) and ZDD_(ij) are the coupling thermal impedances fromthe i_(th) diode to the j_(th) IGBT and j_(th) diode, respectively,ZDI5_(ij) and ZDD5_(ij) are the coupling thermal impedances from thej_(th) diode to the j_(th) IGBT and j_(th) diodes at the sink layer,respectively, and, ZDI6_(ij) and ZDD6_(ij) are the coupling thermalimpedances from the i_(th) IGBT to the j_(th) IGBT and the j_(th) diodesat the ambient layer, respectively.

When multiple IGBTs and diodes are operated on a single module, thetotal temperature increase of one device between its junction to ambientthat is attributable to the other devices is the sum of thecontributions from all module IGBTs and diodes as shown in the followingequations:

$\begin{matrix}{{TI}_{k} = {{\sum\limits_{i = 1}^{6}{{ZII}_{5{ki}} \cdot {PI}_{i}}} + {\sum\limits_{i = 1}^{6}{{ZDI}_{6{ki}} \cdot {PD}_{i}}} + T_{amb}}} & {{Eq}.\mspace{14mu} 9} \\{{TD}_{k} = {{\sum\limits_{i = 1}^{6}{{ZID}_{5{ki}} \cdot {PI}_{i}}} + {\sum\limits_{i = 1}^{6}{{ZDD}_{6{ki}} \cdot {PD}_{i}}} + T_{amb}}} & {{Eq}.\mspace{14mu} 10}\end{matrix}$where, TI_(k) is the junction temperature of the k_(th) IGBT, TD_(k) isthe junction temperature of the k_(th) diode, PI_(k) is the power lossof the k_(th) IGBT, PD_(k) is the power loss of the k_(th) diode andT_(amb) is the ambient temperature.

The IGBT and diode temperatures are generally predicted using atemperature value identified by the NTC sensor 24 (see FIG. 2).Unfortunately, as illustrated in FIG. 2, the NTC sensor is separatedfrom the device junctions and thus thermal impedance exists between thejunctions and the NTC sensor such that the sensor generated value doesnot accurately reflect the junction temperature. For this reason, theinfluence of the thermal impedance between the device junctions and theNTC sensor should be accounted to accurately predict junctiontemperature. Using a method similar to the method described above, theNTC temperature can be predicted by knowing device losses and thedistance of the devices to the NTC sensor. A two layer RC circuit thatrepresents the coupling thermal impedance from the IGBT and diodedevices mounted on a sink to the NTC sensor is shown in FIG. 6 where thesink and ambient layer impedances are represented by series RCsub-circuits including R_(L3) and C_(L3) as well as R_(L4) and C_(L4).The coupling thermal impedances ZIN_(i) and ZDN_(i) of the i_(th) IGBTand the i_(th) diode to the NTC sensor, respectively, can be expressedby the following two equations:ZIN _(i) =ZIN _(5i) +ZIN _(6i)  Eq. 11ZDN _(i) =ZDN _(5i) +ZDN _(6i)  Eq. 12where ZIN_(5i) and ZIN_(6i) are the coupling thermal impedance from thei_(th) IGBT to the NTC sensor at sink and ambient layers and ZDN_(5i)and ZDN_(6i) are the coupling thermal impedance from the i_(th) diode tothe NTC sensor at sink layers and ambient layers, respectively.

The total temperature increase T_(ntc) of an NTC can be expressed as.

$\begin{matrix}{T_{ntc} = {{\sum\limits_{i = 1}^{6}{{ZIN}_{5i} \cdot {PI}_{i}}} + {\sum\limits_{i = 1}^{6}{{ZIN}_{6i} \cdot {PD}_{i}}} + T_{amb}}} & {{Eq}.\mspace{14mu} 13}\end{matrix}$

Combining Equations 3 through 13, the following equations can beformulated for directly calculating IGBT and diode temperature estimatesusing a thermal impedance matrix and known device power losses:TI=ZII·PI+ZDI·PD+Tamb  Eq. 14TD=ZDI·PI+ZDD·PD+Tamb  Eq. 15where:

TI=[TI₁ TI₂ . . . TI₆]^(T) is an IGBT temperature vector,

TD=[TD₁ TD₂ . . . TD₆]^(T) is a diode temperature vector,

PI=[PI₁ PI₂ . . . PI₆]^(T) is an IGBT power losses vector,

PD=[PD₁ PD₂ . . . PD₆]^(T) is a diode power losses vector, and

T_(amb)=[T_(amb1) T_(amb2) . . . T_(amb6)]^(T) is an ambient temperaturevector.

In Equations 14 and 15, ZII, ZDI, ZID, and ZDD are each 6×6 matricesthat represent the coupling thermal impedance matrix from the IGBTs tothe IGBTs, from the IGBTs to the diodes, from the diodes to IGBTs andfrom the diodes to the diodes, respectively. The 6×6 matrices can beexpressed in the following form:

${ZII} = \begin{bmatrix}{ZII}_{11} & {ZII}_{12} & \ldots & {ZII}_{16} \\{ZII}_{21} & {ZII}_{22} & \ldots & {ZII}_{26} \\\ldots & \ldots & \ldots & \ldots \\{ZII}_{61} & {ZII}_{62} & \ldots & {ZII}_{66}\end{bmatrix}$ ${ZDI} = \begin{bmatrix}{ZDI}_{11} & {ZDI}_{12} & \ldots & {ZDI}_{16} \\{ZDI}_{21} & {ZDI}_{22} & \ldots & {ZDI}_{26} \\\ldots & \ldots & \ldots & \ldots \\{ZDI}_{61} & {ZDI}_{62} & \ldots & {ZDI}_{66}\end{bmatrix}$ ${ZID} = \begin{bmatrix}{ZID}_{11} & {ZID}_{12} & \ldots & {ZID}_{16} \\{ZID}_{21} & {ZID}_{22} & \ldots & {ZID}_{26} \\\ldots & \ldots & \ldots & \ldots \\{ZID}_{61} & {ZID}_{62} & \ldots & {ZID}_{66}\end{bmatrix}$ ${ZDD} = \begin{bmatrix}{ZDD}_{11} & {ZDD}_{12} & \ldots & {ZDD}_{16} \\{ZDD}_{21} & {ZDD}_{22} & \ldots & {ZDD}_{26} \\\ldots & \ldots & \ldots & \ldots \\{ZDD}_{61} & {ZDD}_{62} & \ldots & {ZDD}_{66}\end{bmatrix}$where ZII_(ij), ZDI_(ij), ZID_(ij), and ZDD_(ij) are the impedancesdiscussed above.

It should be appreciated that Equations 14 and 15 are relatively complexand would be somewhat burdensome to solve using a standard drivemicro-processor. For this reason, hereafter, several assumptions aremade that enable simplification of Equations 14 and 15. To this end,based on the locations of the IGBTs and diodes as shown in FIG. 2,several assumptions and approximations can be made to simplify theequations. First, the coupling impedance and the self thermal impedancein the ambient layers are equal such that:ZII _(6ij) =ZDI _(6ij) =ZID _(6ij) =ZDD _(6ij) =ZIN _(6i) =ZDN _(6i) =Z_(amb)  Eq. 16

Second, at the sink layer, the coupling impedances associated withneighboring or adjacent devices that are an identical or similardistance away from one device should be approximately equal or at leastsimilar enough that the differences between these coupling impedancesare trivial and can be neglected such that:ZII _(5ij)|_(|i−j|=1) =ZDD _(5ij)|_(|i−j|=1) =ZID _(5ij)|_(|i−j|=1)=ZDI| _(|i−j|=1) =Z ₁  Eq. 17ZII _(5ij)|_(|i−j|=2) =ZDD _(5ij)|_(|i−j|=2) =ZID _(5ij)|_(|i−j|=2)=ZDI| _(|i−j|=2) =Z ₂  Eq. 18where Z₁ corresponds to devices that are one device away (i.e., that areadjacent) from a specific device for which junction temperature is beingpredicted and Z₂ corresponds to devices that are two devices away fromthe specific device for which junction temperature is being predicted.

Third, at the sink layer, the coupling impedances between devices thatare separated by relatively large distances can be neglected. Forexample, while adjacent devices and devices that are one or two devicedimensions (i.e., one or two device widths) away from a first device mayeffect the temperature of the first device, devices that are three ormore device dimensions away from the first device will only have anegligible effect on the temperature of the first device and thus theeffect can be ignored without significantly effecting the finaltemperature prediction. Here, for instance, where the effects of devicesthat are more than two devices away from a device for which the junctiontemperature is being predicted are ignored, the relationshipsrepresented by the following equation will be substantially accurate:ZII _(5ij)|_(|i−j|>2) =ZDI _(5ij)|_(|i−j|>2) =ZID _(5ij)|_(|i−j|>2) =ZDD_(5ij)|_(|i−j|>)=0 ZIN _(5i|i>2) =ZDN _(5i|i>2)=0  Eq. 19

Combining Equations 14 and 16 through 19, the following IGBT temperaturevector equation can be formed:TI=[(ZI ₀ +Z _(jc))I ₆ +Z]PI+[ZDI ₀ ·I ₆ +Z]PD+P·Zamb·A+Tamb  Eq. 20where:

-   I₆ is the 6-by-6 identity matrix,-   P is the total losses of all IGBTs and diodes,-   A=[1 1 . . . 1]^(T) is a 6-by-1 vector, and-   Z is a coupling thermal impedance matrix that has the form:

$Z = \begin{bmatrix}0 & Z_{1} & Z_{2} & 0 & 0 & 0 \\Z_{1} & 0 & Z_{1} & Z_{2} & 0 & 0 \\Z_{2} & Z_{1} & 0 & Z_{1} & Z_{2} & 0 \\0 & Z_{2} & Z_{1} & 0 & Z_{1} & Z_{2} \\0 & 0 & Z_{2} & Z_{1} & 0 & Z_{1} \\0 & 0 & 0 & Z_{2} & Z_{1} & 0\end{bmatrix}$

Similarly, Equations 15 through 19 can be combined to yield thefollowing diode temperature vector equation:TD=[(ZD ₀ +ZD _(jc))I ₆ +Z]PI+[ZID ₀ I ₆ +Z]PD+PZamb·A+Tamb  Eq. 21

Equations 13 and 16 through 19 can be combined to simplify the NTCtemperature equation as follows:

$\begin{matrix}{T_{ntc} = {{\sum\limits_{i = 1}^{3}{{ZIN}_{5i}{PI}_{i}}} + {\sum\limits_{i = 1}^{3}{{ZDN}_{5i}{PD}_{i}}} + {{PZamb} \cdot A} + T_{amb}}} & {{Eq}.\mspace{14mu} 22}\end{matrix}$

Upon examining equations 20-22, it should be recognized that the thermalimpedance in the ambient layer will be cancelled when predicting theIGBT and diode temperatures using the NTC sensor value. However, thecoupling thermal impedance of the sink layer cannot be neglected.

Equations 20 and 22 can be combined to yield the following equation forpredicting the IGBT junction temperatures from the NTC sensortemperature value and power losses:

$\begin{matrix}{{{TI} - {T_{ntc}A}} = {{\left\lbrack {{\left( {{ZI}_{0} + Z_{jc}} \right)I_{6}} + Z} \right\rbrack{PI}} + {\left\lbrack {{{ZDI}_{0} \cdot I_{6}} + Z} \right\rbrack{PD}} - {\left( {{\sum\limits_{i = 1}^{3}{{ZIN}_{5i}{PI}_{i}}} + {\sum\limits_{i = 1}^{3}{{ZDN}_{5i}{PD}_{i}}}} \right) \cdot A}}} & {{Eq}.\mspace{14mu} 23}\end{matrix}$

Similarly, equations 20 and 21 can be combined to yield the followingequation for predicting the diode junction temperatures from the NTCsensor temperature value and the power losses:

$\begin{matrix}{{{TD} - {T_{ntc}A}} = {{\left\lbrack {{\left( {{ZD}_{0} + {ZD}_{jc}} \right)I_{6}} + Z} \right\rbrack{PI}} + {\left\lbrack {{{ZID}_{0}I_{6}} + Z} \right\rbrack{PD}} - {\left( {{\sum\limits_{i = 1}^{3}{{ZIN}_{5i}{PI}_{i}}} + {\sum\limits_{i = 1}^{3}{{ZDN}_{5i}{PD}_{i}}}} \right) \cdot A}}} & {{Eq}.\mspace{14mu} 24}\end{matrix}$

As seen in Equations 23 and 24, the ambient layer impedance and theambient temperature from Equations 21 and 22 both cancel when Equations23 and 24 are formulated.

The thermal impedances in Equation 23 and 24 can be directly calculatedby applying pulsed current through each IGBT and each diode device onthe module. For example, each IGBT and diode device may be injected witha 100 A, 250 second current pulse and the temperature of the deviceduring the pulse can be directly measured and recorded. Thecorresponding thermal impedances can then be calculated by a curvefitting program (e.g., a program built up using Matlab which is owned byMathWorks, Inc., or some similar type of software).

Referring again to FIG. 2 and also to FIG. 7, the data plotted in FIG. 7shows the experimental result of the real temperature and the curvefitted temperature of third diode D3 when the third diode is powered.FIG. 8 shows the experimental result of the real temperature and thecurve fitted temperature of fourth IGBT S4 when the fourth diode D4 ispowered. In FIG. 8 it can be seen that the temperature of diode D4clearly effects the temperature of adjacent IGBT S4. FIG. 9 shows theexperimental result of the real temperature and the curve fittedtemperature of third IGBT S3 when the fourth IGBT S4 is powered. In FIG.9 it can be seen that the temperature of IGBT S4 clearly effects thetemperature of adjacent IGBT S3.

In the curve fitting program used to generate the fitted curves in FIGS.7 through 9, a two layer model was used to approximate the coupling andself thermal impedance of each device. Examining FIGS. 7 through 9 theeffectiveness of the two layer thermal impedance model is clearlyverified.

Exemplary coupling impedance values determined using the curve fittingsoftware and Equations 23 and 24 are listed in tables 2, 3 and 4 withthe thermal impedance values of the sink layer shown in Table 2, thecoupling thermal impedance of the ambient layer shown in Table 3 and thecoupling thermal impedance values from the IGBTs and diodes to the NTCsensor shown in table 4.

TABLE 2 ZI₀ ZD₀ ZDI₀ Z₁ Z₂ R(k/w) 0.1308 0.15 0.0885 0.047 0.02 t (s)1.5 2.2 1.5 1.5 2

TABLE 3 Distance ZI₀ ZD₀ Z₁ Z₂ Z₃ Z₄ R (k/w) 0.055 0.055 0.055 0.050.036 0.032 t (s) 46 46 60 60 80 80

TABLE 4 Sink Layer ZDN₅₁ ZDN₅₂ ZDN₅₃ ZIN₅₁ ZIN₅₂ ZIN₅₃ R (k/w) 0.0840.054 0.017 0.033 0.038 0.005 t (s) 1.45 2.17 10 3.54 3.85 2.00

Using tables 1, 2 and 3, the self thermal resistance of the IGBTs can becalculated by adding the four IGBT related resistances in Table 1 andthe resistances associated with the ZI₀ sink and ambient layers inTables 2 and 3 to, in the present example, yield the following value:R_(II)=0.3657 k/w  Eq. 25

Similarly, referring again to Tables 1, 2 and 3, the self thermalresistance of the diode devices can be calculated by adding the fourdiode related resistances in Table 1 and the resistances associated withthe ZD₀ sink and ambient layers in Tables 2 and 3 to, in the presentexample, yield the following value:R_(DD)=0.545 k/w  Eq. 26

Comparing the values expressed in Equations 25 and 26 to the couplingthermal impedance values (e.g., Z₁, Z₂, Z₃, etc.) shown in tables 2 and3, the following conclusions can be made. First, in the sink layer, thecoupling thermal impedances Z₁, Z₂, between neighbor chips are muchsmaller than the self impedance and therefore impedance values Z₁, Z₂can have at least some error without significantly effecting temperatureprediction accuracy. Second, the coupling thermal impedance in theambient layer (see Table 3) is trivial when compared to the selfimpedance and therefore a uniform thermal impedance can be assumedwithout significantly effecting temperature prediction accuracy. Theseassumptions are consistent with an understanding that Equations 23 and24 above are relatively accurate.

Results

The temperature prediction Equations 23 and 24 have been used to predictthe junction temperatures of module devices where the devices have beenoperated under DC conditions. During testing, four models wereinvestigated and compared with each other so that the accuracy ofEquations 23 and 24 could be ascertained.

The first model is called “R_(jc) model” in which a uniform casetemperature was assumed. In this case the NTC temperature is assumed tobe equal to the case layer.

The second model is referred to as the “No neighbor” model because, asthe label implies, the model did not account for the thermal couplingbetween neighboring or adjacent IGBTs and diodes. Thus, in this case, itwas assumed that Z₁ and Z₂ each were equal to zero in the temperatureprediction equations above (i.e., in equations 23 and 24 and the Zmatrix as shown in Equation 20). However, in this second model, thethermal impedances of the devices themselves (e.g., ZII₀, ZDD₀) wereconsidered.

The third model is referred to as the “one neighbor” model because themodel includes equations that consider the coupling thermal impedancebetween the devices that are closest or immediately adjacent a specificdevice for which the junction temperature is being predicted. Here, thecoupling thermal impedance is neglected when a device is separated froma device for which the temperature is being predicted by at least oneother device. Thus, in the third model Z₂ was set equal to zero inEquations 20, 23 and 24.

The fourth model is referred to as the “two neighbor” model because themodel includes equations that consider the coupling thermal impedancebetween devices immediately adjacent a device for which temperature isbeing predicted as well as devices that are adjacent the immediatelyadjacent devices (i.e., devices that are no more than two devices awayfrom the device for which temperature is being predicted. Here, thecoupling thermal impedance is neglected when a device is separated froma device for which the temperature is being predicted by at least twoother device.

During testing, an inverter module akin to module 14 illustrated in FIG.2 was operated under the following conditions:

DC bus voltage: 300 V Switching frequency: 2 kHz~10 kHz Load currentamplitude: 20 A~100 A Load current angle: 0°~330° Ambient temperature:22° C.

The maximum temperature increase of the hottest device during testingwas approximately 110° C.

Table 5 shows the maximum and minimum temperature prediction errorsMax(T_(err)) and Min(T_(err)), respectively, for all IGBTs and diodesfor each of the four models. In Table 5, T_(err) is defined as thepredicted junction minus the tested temperature and is positive when thepredicted temperature is higher than the tested result. To increaseaccuracy of the power losses calculation, the voltage drops of the IGBTsand diodes were calculated in detail by considering the influence of thejunction temperature. The resulting temperature errors are shown inTable 5. In addition, the maximum voltage drops were also calculatedusing a worst case 125° Celsius V/I curve and neglecting the junctiontemperature influence on the voltage drops. The resulting maximumtemperature error values when the 125° Celsius curves were assumed arenot shown here but it is noted that the maximum values were almostidentical to those shown in table 5 where the junction temperatureeffect was considered. Thus, it can be concluded that the 125° Celsiuscurves can be used without appreciably effecting prediction accuracy.

TABLE 5 T_(err) R_(jc) model No neighbor One neighbor Two neighborMax(T_(err)) 20.9 10.4 10.1 10.2 Min(T_(err)) −32.1 −18.2 −8.5 −7.9

From table 5 it can be seen that the R_(jc) model results in a largemaximum temperature prediction error (30° C.). Thus, it is not possibleto predict the junction temperature accurately by assuming a uniformcase temperature and NTC temperature under DC condition.

The “one neighbor model” is the simplest model that yields relativelyaccurate results and that the “two neighbor model” is not significantlymore accurate than the one neighbor model.

There are still around ±10° C. maximum temperature prediction errorsafter accounting for the thermal interface between neighboring devices.These errors are mostly generated by fluctuations of the voltage forwarddrop, switching losses, current/voltage measurement, non-ideal geometryof different chips and thermal resistance.

Referring again to Tables 2, 3 and 4, after values required to solveEquations 23 and 24 have been determined via Mathlab or some othersimilar product, the values can be used in Equations 23 and 24 duringnormal operation of the associated module 14 (see again FIG. 2) topredict device temperatures.

It should be appreciated that a thermal model has been developed for asix-pack insulated gate bipolar transistor (IGBT) power module operatingas a three phase voltage source inverter. With this method, two morethermal layers are added to the system to predict the chip temperaturesfrom the NTC sensor value. The inventive model increases the temperatureprediction accuracy when the inverter operates at zero or low outputfrequency. The model is not complicated and can be easily integratedinto a micro-controller programs for dynamic temperature prediction.

One or more specific embodiments of the present invention have beendescribed above. It should be appreciated that in the development of anysuch actual implementation, as in any engineering or design project,numerous implementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness related constraints, which may vary from one implementation toanother. Moreover, it should be appreciated that such a developmenteffort might be complex and time consuming, but would nevertheless be aroutine undertaking of design, fabrication, and manufacture for those ofordinary skill having the benefit of this disclosure.

Thus, the invention is to cover all modifications, equivalents, andalternatives failing within the spirit and scope of the invention asdefined by the following appended claims. For example, while usefulalgorithms are described above wherein the thermal coupling betweenadjacent and one removed devices is used to predict the junctiontemperature of a specific device, in some cases other algorithms may beused that account for thermal coupling between other module devices orindeed all module devices mounted to the same sink member.

In addition, while the invention is described above in the context of asix-pak module, it should be appreciated that the invention is useful inthe context of other modules such as four-paks, twelve-paks, eighteenpaks and so on.

Moreover, while the invention is described above in the context of amodule that includes IGBTs, the invention is applicable to other modulesthat include other device types such as, for instance, MOSFETS, IGCTs,etc.

Furthermore, while the NTC is described above as being located on themodule case, in at least some embodiments the NTC may be locatedelsewhere and still very close to the module and satisfactory resultswill still occur.

At this point it should be appreciated that the circuits illustrated inFIGS. 3-6 do not represent exact system impedances but rather arepredictive in nature. Similarly, the equations based on FIGS. 3-6 andmodified above based on various assumptions are not precise and insteadare predictive in nature.

To apprise the public of the scope of this invention, the followingclaims are made:

1. A method for predicting junction device temperature of at least afirst switching device in a power conversion module that includes aplurality of switching devices, the method comprising the steps of:providing a processor that runs a program to perform the steps of:during switching activity, identifying at least one operatingcharacteristic of the first switching device; and solving an equationthat uses the identified operating characteristic to predict thejunction device temperature of the first switching device where theequation solved is a function of the distance of the first switchingdevice with respect to the other switching devices in the plurality. 2.The method of claim 1 wherein the operating characteristic is the powerloss of the first switching device.
 3. The method of claim 2 wherein theequation accounts for power losses of switching devices adjacent thefirst switching device as well as cross thermal impedance between theadjacent devices and the first switching device and wherein the methodfurther includes the steps of identifying a cross thermal impedancebetween a switching device adjacent the first switching device and thefirst switching device and, during switching activity, identifying thepower losses of switching devices adjacent the first switching device.4. The method of claim 3 wherein the switching devices adjacent thefirst switching device are neighboring devices and wherein the equationaccounts for power losses of switching devices adjacent the neighboringdevices as well as cross thermal impedance between the devices adjacentthe neighboring devices and wherein the method further includes thesteps of identifying a cross thermal impedance between a switchingdevice adjacent a neighboring switching device and the first switchingdevice and, during switching activity, identifying the power losses ofswitching devices adjacent the first switching device.
 5. A method foruse with a module that includes a plurality of electronic switchingdevices, the method for predicting the junction temperature of at leasta first of the switching devices, the method comprising the steps of:providing a processor for identifying a power loss value of at least asecond of the switching devices; providing a database in which is storeda cross thermal impedance value indicative of the thermal impedancebetween the first and second switching devices; and the processor usingthe power loss value of the at least a second switching device and thecross thermal impedance to predict the temperature of the firstswitching device junction.
 6. The method of claim 5 wherein the databaseincludes cross thermal impedance values for a sub-set of the devices inaddition to the second device that are indicative of the thermalimpedance between each of the other devices and the first device, themethod further including the steps of identifying power loss values foreach of the sub-set of devices and using all of the power loss valuesand the cross thermal impedance values to predict the temperature of thefirst switching device junction.
 7. An apparatus for predicting junctiondevice temperature of at least a first switching device in a powerconversion module that includes a plurality of switching devices, theapparatus comprising: a processor that runs a program to perform thesteps of: during switching activity, identifying at least one operatingcharacteristic of the first switching device; and solving an equationthat uses the identified operating characteristic to predict thejunction device temperature of the first switching device where theequation solved is a function of the distance of the first switchingdevice with respect to the other switching devices in the plurality. 8.The apparatus of claim 7 wherein the operating characteristic is thepower loss of the first switching device.
 9. The apparatus of claim 8wherein the equation accounts for power losses of switching devicesadjacent the first switching device as well as cross thermal impedancebetween the adjacent devices and the first switching device and whereinthe processor further performs the steps of identifying a cross thermalimpedance between a switching device adjacent the first switching deviceand the first switching device and, during switching activity,identifying the power losses of switching devices adjacent the firstswitching device.
 10. The apparatus of claim 9 wherein the switchingdevices adjacent the first switching device are neighboring devices andwherein the equation accounts for power losses of switching devicesadjacent the neighboring devices as well as cross thermal impedancebetween the devices adjacent the neighboring devices and wherein theprocessor further performs the steps of identifying a cross thermalimpedance between a switching device adjacent a neighboring switchingdevice and the first switching device and, during switching activity,identifying the power losses of switching devices adjacent the firstswitching device.
 11. An apparatus for use with a module that includes aplurality of electronic switching devices, the apparatus for predictingthe junction temperature of at least a first of the switching devices,the apparatus comprising: a database in which is stored a cross thermalimpedance value indicative of the thermal impedance between the firstand second devices; and a processor that runs a program to perform thesteps of: identifying power loss of at least a second of the switchingdevices; using the power loss value of the at least a second switchingdevice and the cross thermal impedance to predict the temperature of thefirst switching device junction.
 12. The apparatus of claim 11 whereinthe database includes cross thermal impedance values for a sub-set ofthe devices in addition to the second device that are indicative of thethermal impedance between each of the other devices and the firstdevice, the processor further identifying power loss values for each ofthe sub-set of devices and using all of the power loss values and thecross thermal impedance values to predict the temperature of the firstswitching device junction.
 13. A system comprising: a power conversionmodule that includes a plurality of switching devices; and a processorthat runs a program to perform the step of: predicting a junctiontemperature of a first of the plurality of switching devices by solvingan equation that is a function of the distance of the first switchingdevice with respect to other switching devices of the power conversionmodule.
 14. The system of claim 13 wherein the equation uses anidentified operating characteristic of the first switching device topredict the junction temperature.
 15. The system of claim 14 wherein theoperating characteristic is the power loss of the first switchingdevice.
 16. A system comprising: a power conversion module that includesa plurality of switching devices; a processor that runs a program toperform the steps of: predicting a junction temperature of a first ofthe plurality of switching devices; identifying a cross thermalimpedance between the first switching device and a switching deviceadjacent the first switching device; and during switching activity,identifying power losses of other switching devices adjacent the firstswitching device using an equation that is a function of the distance ofthe first switching device with respect to the other switching devices.